1. Field of the Invention
The present invention relates to image-forming apparatuses such as video cameras and electronic still cameras, and in particular, relates to optical devices using electroactive polymer actuators.
2. Description of the Related Art
There has been significant size reduction of video cameras and electronic still cameras in recent years. At the same time, ultrasmall camera units for cellular phones and the like have been developed in succession, and the demand for smaller image-capturing lens units is increasing.
To date, electromagnetic motors have been generally used as actuators for driving movable lenses and light-adjusting diaphragms for autofocusing, zooming, stabilizing images, and adjusting light amounts in video cameras and electronic still cameras. A plurality of electromagnetic motors are installed in a lens barrel, and a reduction in space occupied by the motors in the lens barrel is required as the lens becomes smaller with reduction in image size.
The electromagnetic motors include magnets and coils. Force that is generated by energizing the coils in magnetic fields in accordance with Fleming's rule and mutually acts on the magnets and coils is utilized as a driving force. In order to reduce the size of the actuators, the driving force per volume of the actuators needs to be increased. To that end, the current passing through the electromagnetic actuators and the magnetic fields generated by the magnets need to be increased.
Specifically, the wire diameter of the coils may be increased so as to allow passage of higher current, the winding number of the coils may be increased, or the size of the magnets may be increased so as to increase magnetic flux density. However, each measure may cause an increase in the size of the motors, and does not match the tendency toward smaller image-forming apparatuses in recent years.
When a large load is driven by a small driving force, both a decelerating mechanism and a mechanical lever are utilized in general. When a decelerating mechanism is employed, driving noise of the motor and noise of gears of the decelerating mechanism are disadvantageously generated since the motor drives at a speed higher than that of the load. Moreover, mechanical components added to the decelerating mechanism and the lever may prevent size reduction and low costs, and may impair durability due to mechanical sliding portions thereof.
Recently, research studies on high-polymer materials having large strain and output force per volume have been lively conducted with the aim of using such materials as artificial muscle and the like. Actuators utilizing these materials directly transmit the strain of the materials to loads so as to drive the loads by using the markedly large strain of the materials compared with that of known piezoelectric materials such as lead zirconate titanate (PZT), and do not require decelerating mechanisms or the like.
These materials include those in a category referred to as so-called electroactive polymers, which typically include dielectric elastomers, ferroelectric polymers, liquid-crystal elastomers, electrostrictive polymers, and the like (In Electro Active Polymers (EAP) as Artificial Muscles, Reality Potential and Challenges, 2nd ed.; Y. Bar-Cohen, Ed., SPIE Press: Bellingham, Wash., 2004; pp 22-31).
In these materials, dielectric elastomers include those of an acrylic type and of a silicone type. Some acrylic dielectric elastomers having a strain of 380% or more attract considerable attention. Moreover, since the dielectric elastomers have an output force per volume that is one to two orders of magnitude larger than that of known electromagnetic motors, the volume of the actuators is expected to be reduced to one tenth or less that of the electromagnetic motors. Due to the large output force and strain, the above-described decelerating mechanisms and mechanical levers are not required, and thus silent and durable actuators can be realized.
Operating principles of the dielectric elastomers will now be described by taking a flat dielectric elastomer film as an example (J. D. W. Madden; Artificial Muscle Technology: Physical Principles and Naval Prospects; IEEE Journal of Oceanic Engineering; July 2004, Vol. 29, No. 3).
A flat dielectric elastomer film interposed between two electrodes is compressed in the direction of an electric field by an electrostatic force between the electrodes generated when a voltage is applied to the electrodes (Maxwell's stress), and at the same time, is expanded in the direction perpendicular to the electric field so as to generate a pressure P. The pressure P can be represented by Expression 1 shown below (In Electro Active Polymers (EAP) as Artificial Muscles, Reality Potential and Challenges, 2nd ed.; Y. Bar-Cohen, Ed., SPIE Press: Bellingham, Wash., 2004; pp 535-538). The pressure P is utilized as the driving force of an actuator. As is clear from Expression 1, materials having a larger dielectric constant, a smaller gap between the electrodes, and a larger driving voltage can increase the driving force.P=∈r∈0(V/t)2  (1)
where, ∈r, ∈0, V, and t indicate the relative dielectric constant, the dielectric constant in a vacuum (8.85×10−12 F/m), the voltage between the electrodes, and the gap between the electrodes, respectively.
Moreover, the relationship between the displacement of the film and the load can be represented by Expression 2 shown below (In Electro Active Polymers (EAP) as Artificial Muscles, Reality Potential and Challenges, 2nd ed.; Y. Bar-Cohen, Ed., SPIE Press: Bellingham, Wash., 2004; pp 535-538).Δl=l(0.5P−F/wt)/Y  (2)
where, Δl, l, P, F, w, t, and Y indicate the displacement of the actuator in a direction along which the force is obtained (extension of the film), the initial length of the film, the generated pressure (Expression 1), the load, the width of the film, the thickness of the film, and Young's modulus, respectively.
The electrodes composed of an elastic material such as carbon are formed on the dielectric elastomer film. Acrylic dielectric elastomers (VHB4910 of 3M make, for example) and silicone dielectric elastomers are commercially available.
U.S. Pat. No. 6,891,317 discloses a cylindrical actuator including a thin film of such a dielectric elastomer wound around a helical compression spring. This circular actuator functions as a one-dimensional linear actuator that extends or contracts in the axial direction and also as a two-dimensional bending actuator whose top portion bends. Furthermore, a so-called push-pull actuator including two such cylindrical polymer actuators is described in Science American (October 2003, p 58). The helical compression spring applies a predetermined extension (prestrain) to the dielectric elastomer film in the circumferential direction and in the axial direction beforehand such that the electrostatic breakdown strength of the film is increased. Higher voltage can be applied to the film as the strength thereof is increased, and as a result, a small and reliable actuator having an increased driving force can be realized. The electrostatic breakdown strength is 110 to 350 MV/m for silicone dielectric elastomers, and 125 to 440 MV/m for acrylic dielectric elastomers (J. D. W. Madden; Artificial Muscle Technology: Physical Principles and Naval Prospects; IEEE Journal of Oceanic Engineering; July 2004, Vol. 29, No. 3).
Moreover, U.S. Pat. No. 6,809,462 discloses an application of a dielectric elastomer as a displacement-detecting sensor by detecting changes in electrical properties such as the capacitance and resistance of the elastomer caused by the deformation of the elastomer. However, methods for detecting changes in capacitance and the like are not limited to this, and other methods using, for example, high-frequency resonators of an RCA type are widely known. Moreover, changes in electrical properties including resistance can be detected using known detecting circuits. Furthermore, composites including actuators and sensors deposited on each other using piezoelectric elements are also widely known.
Application of the cylindrical actuator according to U.S. Pat. No. 6,891,317 to an actuator for driving a lens leads to the following problems:                1. Output can only be transmitted via the end portion of the cylinder.        2. The helical compression spring is disposed inside the cylindrical film.        3. No guiding mechanism in the direction along which the force is obtained is provided.        
The first problem is that the layout of the actuator installed inside a lens barrel is performed with low flexibility. Moreover, when the lens is driven by the cylindrical actuator whose force is output from the end portion, a space is required on one side of the lens for the cylindrical actuator. Therefore, even when the size of the cylindrical actuator is reduced compared with electromagnetic actuators, the merit may not be utilized in terms of layout.
The second problem is that the dielectric elastomer film is disposed on the outer circumference of the spring, and the film is deformed along the outside shape of the spring due to the small stiffness of the film. Furthermore, the spring and the film relatively slide on each other when the film extends or contracts. The deformation and the sliding may exert detrimental effects on driving performance characteristics such as hysteresis and repeatability, durability, and reliability characteristics such as electrostatic breakdown strength.
Moreover, the total length of the rolled actuator is determined by the balance of the force between the spring and the film, and the tolerance and the temperature of the film and the spring have profound effects on the total length since the film and the spring are in contact with and slide on each other as described above. Thus, the control of tolerance of the total length becomes difficult. This means that characteristics such as strain and driving force may vary more widely with the variation of the total length.
The third problem is that driving force in a predetermined direction is not stabilized since no guiding mechanism in the direction along which the force is obtained is provided. This may also exert detrimental effects on the driving performance as in the second problem.